The present invention relates to reduction in a brush temperature of a brush motor for an electric power steering system, which has a cylindrical commutator a plurality of commutator segments of which are secured by the resin and a brush that slidably contact to outer peripheral surfaces of the commutator segments of this commutator.
In the brush motor for the electric power steering system in the prior art, in order to reduce temperature rise of the brush, heat generation is suppressed by increasing a reduction rate of the current that is supplied to the motor, and also a heat capacity is improved by increasing a size of the shape of the motor itself, and in addition the heat transferring member that is brought into tight contact with the brush holder to absorb/transfer the heat being generated in the brush is provided to lower the temperature rise of the brush (see Patent literature 1).
The Unexamined Japanese Patent Application Publication No.2002-153022 (Paragraphs [0005], [0007], FIG. 1, FIG. 2)
In the brush motor for the electric power steering system in the prior art, in order to avoid the overheat of the motor, the heat generation is suppressed by increasing the reduction rate of the current, which is supplied to the motor, to limit the supply current. For this reason, the assist torque applied by the motor is reduced in a relatively short time, and thus the steering torque required of the driver is increased. Therefore, in some cases the steering feeling is impaired particularly at the time of low speed driving of the vehicle, and the like. Further, if the heat capacity is improved by increasing a size of the shape of the motor itself, the moment of inertia and the loss of torque are increased in addition to reduction of the mounting easiness and increase of the noise with the increase in size of the motor. Therefore, the steering feeling and the turning-back of the steering wheel are impaired. In addition, if the temperature rise is reduced by bringing the heat transferring member into tight contact with the brush holder, the problems such as the increase of the number of parts, the increase in size and the increase in the noise due to the addition of the heat transferring member, etc. are caused.
In the brush motor for the electric power steering system, when the user steers without running, when the user drives a car into a garage, etc., a current that is larger than that at the steering in the normal running must be supplied. Therefore, the large current is supplied in the situation that the rotation of the motor is stopped or the motor is rotated at the very low speed. Thus, it becomes the important subject to lower the brush temperature in such peculiar used situation. As a result, another solving means that is suitable for the brush motor for the electric power steering system to lower the brush temperature is requested.
The present invention is made to overcome the above problems, and it is an object of the present invention to get a brush temperature lowering means suitable for a brush motor for an electric power steering system, which is capable of suppressing the temperature rise of the brush portion by thermally transferring heat generated in the brush portion effectively to the armature side, so that the temperature rise of the brush can be suppressed easily without increase in size of the motor if the number of parts is small, the steering feeling can be improved by reducing correspondingly the reduction rate of the current that is supplied to the motor or by increasing the supply current, the noise can be reduced, etc.
A brush motor for an electric power steering system according to the present invention, comprises a four-pole field portion fixed to an inner wall surface of a yoke; an armature having windings which are wound around a core with 22 slots to constitute a lap winding, hooks which are formed on one ends of 22 commutator segments respectively and on which the windings are hooked, and a cylindrical commutator which is constructed by molding integrally the commutator segments with a resin, whereby convex portions for preventing fling-out of the commutator segment are provided on an inner peripheral surface side of each commutator segment to extend over an almost full length in an axial direction; and four brushes which contact slidably to outer peripheral surfaces of the commutator segments; wherein, when the commutator to which a sliding contact portion of the brush contacts is sectioned perpendicularly to an axial direction, a following relationship is satisfied.
0.18xe2x89xa6(Axc3x97N)/(Dxc3x97D)xe2x89xa60.23
where A is a sectional area (mm2) of one commutator segment, N is a number of the commutator segments, and D is a diameter (mm) of outer peripheral surfaces of the commutator segments.
Further, a brush motor for an electric power steering system according to the present invention, comprises a four-pole field portion fixed to an inner wall surface of a yoke; an armature having windings which are wound around a core with 22 slots to constitute a lap winding, hooks which are formed on one ends of 22 commutator segments respectively and on which the windings are hooked, and a cylindrical commutator which is constructed by molding integrally the commutator segments with a resin, whereby one leg portion is provided on an inner peripheral surface side of each commutator segment to extend over an almost full length in an axial direction and convex portions for preventing fling-out of the commutator segment are provided to one end of the leg portion; and four brushes which contact slidably to outer peripheral surfaces of the commutator segments; wherein, when the commutator to which a sliding contact portion of the brush contacts is sectioned perpendicularly to an axial direction, a following relationship is satisfied.
0.10xe2x89xa6T/Dxe2x89xa60.14
where T is a thickness (mm) of the commutator segment in a radial direction, and D is a diameter (mm) of outer peripheral surfaces of the commutator segments.